Monitoring Solar Panel Performance

--Blog post written by Bob

We now have two identical solar panels in operation--one is on top of the hard dodger and the other is on top of the forward section of the bimini (as described in my blog post dated April 6, 2014).  Each panel has its own voltage controller with a green light that varies with the output of the solar panel--the light varies from slowly flashing to solid green but there is no true measurement of the solar panel's output.  The engineer in me wanted to be able to measure the voltage output of each panel. The voltage generated is a rough indication of how much energy is being converted from sunlight to the voltage controller.  If one solar panel was in a shadow it should show a lower voltage being produced--this would tell me to eliminate the shadow if possible.

On Amazon, I found tiny digital voltmeters made by Drok for under $7 each--I purchased two.  The voltmeters are rated from 3 volts to 30 volts and require no external power source.  The solar panels produce up to 24 volts (open circuit) ahead of the voltage controller--so the voltage range of the tiny digital voltmeters was perfect.

Image of Digital Voltmeter on Amazon

When they arrived, I checked each one of them by holding the wires to a 3-volt watch battery.  The display lit up and clearly showed 3.16 volts on both digital voltmeters.

Testing the New Digital Voltmeter with a Watch Battery

Prior to installing them, I coated the circuit boards on the back with two coats of epoxy to make them moisture resistant. (I accidentally coated the precision adjustment screw in position too...oh well...guess I won't be making any precision adjustments now.)

I mounted each voltmeter in a plastic bracket and mounted the bracket near the solar panel's input to the voltage regulators.  After hooking up the wires to each panel's output wires, I can see how much voltage each panel is producing at any time.  (This is NOT open circuit voltage, one of the measurements used to rate solar panels.  The only value I can gain from these two voltmeters is to determine the output of one panel relative to the other panel at the same time.)

The solar panel mounted on the hard dodger was producing 18.3 volts at the same time the bimini-mounted solar panel was producing 17.5 volts--this was about 9:30 AM.  At about 1:00 PM the dodger-mounted panel was reading 17.6 volts while the bimini-mounted panel was reading 16.3 volts.  At 6 PM, both panels were equal and reading over 17 volts.  (At the present time I have no way of knowing the difference in amps, which is the true measure of power production).

Voltmeters in Use at 9:30 AM

The differences in voltage readings can only be explained by very slight differences in panel inclination (about 5 degrees) and the position of the sun with respect to the position of the boat in the slip. (In the slip, our boat faces SSW.)  It is interesting that the shadow from the boom on the dodger-mounted solar panel has an insignificant effect on voltage, perhaps because it is at least one foot above the panel.

While this was an interesting and inexpensive exercise, I'm not sure that we learned all that much about each solar panel's performance.

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